The Study on Improving Performance of Polymer Solar Cells by Surface Modification at Organic/Inorganic Interface
Date Issued
2010
Date
2010
Author(s)
Wang, Ing-Jye
Abstract
Various technologies designed to generate power by harnessing solar energy have been gaining interest because of the dwindling supplies of natural resources. In the past decade, organic photovoltaics (OPVs) are attractive because of their compatibility with flexible substrates, roll-to-roll processing, low manufacturing costs, and large area applications.
However, the conventional bulk-heterojunction (BHJ) architecture has limitations in device stability because of the acidic, hygroscopic nature of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and oxidation of the Al electrode. Moreover, there exist problems of inherently poor polymer properties, such as the short exciton diffusion length and the relatively low carrier mobility, which limit the usefulness of thick film.
The state-of-the-art device structure is the polymer BHJ, a conjugated polymer blended intimately with a soluble fullerene derivative. This method generates a maximized interface between the donor and acceptor materials within the whole volume of the photoactive layer. Which in turn, creates maximum exciton dissociation and ensures the creation of continuous, preferably short, pathways for charge transport to both of the electrodes. Recently, many studies have shown that the device performance is related to the morphology of the blend film, including the formation of domains from different compositions and the packing of the molecules. Therefore, morphology optimization of the active layer is an essential way to improve power conversion efficiency (PCE).
The aim of this work is to realize a high-efficiency inverted polymer solar cells with surface modification on two sides of the polymer thin film. Our investigation shows that the plasma-etched polymer surface adjacent to the anode was modified into a more hydrophobic P3HT-rich surface, which leads to better interface contact, fine electrode selectivity, and lower series resistance in the device. Although PCBM still aggregates on the top surface, it can be removed with mild oxygen plasma etching. As a result, the power conversion efficiency increased significantly from 3.4 to 4.3% with an enhanced fill factor of 64%.
Another study is on the surface modification of the ZnO thin film adjacent to the cathode. A thin low work function metal film is sandwiched between the ZnO and organic layer to improve the performance for inverted polymer solar cells. This metal film modifies the organic surface for efficient carrier transport and also eliminates carrier accumulation. In cases where the lowest unoccupied molecular orbital level of the electron acceptor in organic layer is below the Fermi level of the metal substrate, the spontaneous charge transfer from the metal substrate to organic layers leads to the Fermi-level pinning. Therefore, the energy difference at the ZnO/organic interface is reduced so that the electrons can be transported to cathode more efficiently. In addition, the electric field generated by the work function difference between anode and cathode is more significant due to the lower Fermi level of Mg than that of ITO or ZnO; thus, the open circuit voltage is enhanced. Here, we report that inverted polymer solar cells have a power conversion efficiency of 4.63% with Jsc=9.61 mA/cm2, Voc=0.82 V, and FF=58.8% under AM 1.5 G (100mW/cm2) irradiation intensity.
Subjects
Polymer Solar Cells
ZnO Thin Film
Inverted Structure
Oxygen Plasma Etching
Solution Processing
SDGs
Type
thesis
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